1. Field of the Invention
The present invention relates generally to mid-range infrared (IR) laser sources. More specifically, the present invention relates to mid-range IR laser sources produced by difference-frequency generating (DFG) optical circuits using bulk crystals.
2. Description of the Background Art
Mid-range IR (2-4 .mu.m) sources are of interest in the field of spectroscopy, pollution monitoring, electronic warfare (EW) applications, etc. Adequate sources in this wavelength range generally do not exist. Filament, or black body emitters, have low, uncollimated power and provide poor spectral resolution. Semiconductor laser sources require low temperature operation and have limited tunability.
Tunable laser-like sources are generally obtained from optical parametric oscillators (OPO's), which have been available for some time. However these usually have kilowatt power thresholds, which require complicated Q-switched lasers. Often, these lasers must be water cooled. OPO's are usually thermally tuned, often up to 180 degrees C; thus tuning is slow. OPO's are generally considered laboratory setups, as opposed to portable instruments.
Difference frequency generation (DFG), i.e., the subtraction of photons from two laser inputs, has also been used for IR generation. Such nonlinear processes must be phase matched for efficient conversion. It will be appreciated that birefringence phase matching in birefringent crystals is typically used. Characteristic outputs have been low (.apprxeq.50 .mu.W) and these outputs not widely tunable due primarily to limitations of bifringence phase matching.
U.S. Pat. No. 5,434,700 discloses an optical wavelength converter formed from semiconductor materials. This patent also discusses a number of other publications which are cited therein, including a reference by Hermann et al., which discusses the use of a lithium niobate material for difference-frequency generation of tunable, mid-infrared radiation, and the Lim et al. reference, which allegedly discloses the use of a periodically poled lithium niobate waveguide for generating infrared radiation by quasi-phase-matched, difference-frequency mixing.
U.S. Pat. No. 5,412,502 discloses a quasi-phase-matching second harmonic generating optical element. Although this patent is directed to second harmonic generation, as opposed to difference-frequency generation, it will be appreciated that such non-linear ferroelectric optical elements can be used for both applications. In particular, this patent discloses a non-linear ferroelectric optical element, which may be lithium niobate, that is periodically poled, and notes that "inclining the substrate allows the wavelength to be adjusted" to compensate for the dispersion of the semiconductor laser.
U.S. Pat. No. 5,504,616 discloses a wavelength conversion device formed by adding a laser-active material to a non-linear optical crystal. In the Background section, it is noted that the same type of nonlinear optical crystals as are used for second harmonic generation can be used for difference-frequency generation when two different wavelengths are input to the crystal.
U.S. Pat. No. 5,506,722 discloses an optical wavelength converting device utilizing a non-linear periodically poled optical device. Of particular interest is the disclosure of the electromagnetic domains formed in the crystal being rotated relative to the crystal faces.
U.S. Pat. No. 5,058,970 discloses a quasi-phase matching optical waveguide. As discussed therein with reference to FIG. 6, where the width and spacing of the electromagnetic domains are respectively uniform, the substrate may be rotated in order to lengthen or shorten the optical path, while still providing efficient generation of a second harmonic output.
It will be appreciated that these patents are generally directed to low power optical waveguide devices not suited to the output power requirements of many industrial and military applications.